In recent years, as a transmission technique for mobile communication systems, an Orthogonal Frequency Division Multiple Access (OFDMA) technique allowing the efficient use of frequency bands has attracted attention. In the OFDMA technique, information is transmitted and received between a base-station apparatus and a mobile machine by using a plurality of sub-channels. A sub-channel is an information transmission channel including sub-carriers of different frequency bands, and is allocated by the base-station apparatus to any of mobile machines in a mobile communication system.
Examples of a technique for the base-station apparatus to allocate a sub-channel to each mobile machine include a Band-Adaptive Modulation and Coding (Band-AMC) technique, a Partial Usage of Sub-channel (PUSC) technique, a Full Usage of Sub-channel (FUSC) technique, and others. In the Band-AMC technique, a sub-channel including a plurality of sub-carriers with frequency bands adjacent to each other (hereinafter, referred to “adjacent sub-channel”) is allocated to one mobile machine.
Specific explanation is now made by using FIG. 16A. In FIG. 16A, an example is depicted in which sub-carriers C11 to C14, C21 to C24, C31 to C34, and C41 to C44 of different frequency bands are used for wireless communication. With such sub-carriers, a base-station apparatus adopting the Band-AMC technique allocates, for example, an adjacent sub-channel CH1 including the sub-carriers C11 to C14 with frequency bands adjacent to each other, to a mobile machine A. Also, for example, the base-station apparatus adopting the Band-AMC technique allocates an adjacent sub-channel CH2 including the sub-carriers C21 to C24 with frequency bands adjacent to each other to a mobile machine B. At this time, the base-station apparatus allocates a high-quality adjacent sub-channel to each mobile machine. In this manner, the Band-AMC technique has advantages such that a user diversity effect can be obtained in an environment where frequency-selective fading occurs, and throughput can be increased with the increase of an adaptive modulation effect.
In the PUSC technique and the FUSC technique, a sub-channel including a plurality of sub-carriers with distributed frequency bands (hereinafter, referred to as “distributed sub-channel”) is allocated to one mobile machine.
Specific explanation is now made by using FIG. 16B. In FIG. 16B, as with the example depicted in FIG. 16A, an example is depicted in which sub-carriers C11 to C14, C21 to C24, C31 to C34, and C41 to C44 are used for wireless communication. With such sub-carriers, a base-station apparatus adopting the PUSC technique or the FUSC technique allocates, for example, a distributed sub-channel CH5 including the sub-carriers C11, C21, C31, and C41 with distributed frequency bands, to the mobile machine A. Also, for example, the base-station apparatus adopting the PUSC technique or the FUSC technique allocates a distributed sub-channel CH5 including the sub-carriers C12, C22, C32, and C42 with distributed frequency bands to the mobile machine B. In the PUSC technique or the FUSC technique, an average quality sub-channel is allocated to each mobile machine. Therefore, the PUSC technique and the FUSC technique have an advantage such that stable wireless quality can be obtained.
Examples of the conventional technologies are disclosed in International Publication Pamphlet No. WO 05/006622, Japanese Laid-open Patent Publication No. 11-298439, Japanese Laid-open Patent Publication No. 2004-350326, Japanese Laid-open Patent Publication No. 2001-358692, and “Design and implementation of Simulator Based on a Cross-Layer Protocol between MAC and PHY layers in a WiBro Compatible IEEE 802.16e OFDMA Systems”, T. kwon, et al., IEEE Communication Magazine, December, 2005, pp. 136-146.
However, in the conventional mobile communication system using the OFDMA technique has problems such that a reception error may occur at any mobile machine and a sub-channel cannot be appropriately allocated to each mobile machine according to the wireless propagation environment where the mobile machine is located.
Specifically, in the Band-AM technique, to allocate a high-quality adjacent sub-channel to each mobile machine, the adjacent sub-channel to be allocated may be frequently changed for a mobile machine whose wireless propagation environment is frequently varied. In this case, wireless information at the time of allocating the adjacent sub-channel and the wireless propagation environment where the mobile machine is located may not match each other. For this reason, the Band-AMC technique has a problem such that a reception error may occur at the mobile machine.
Moreover, in the PUSC technique and the FUSC technique, merely an average wireless quality can be obtained whatever is a distributed sub-channel allocated to each mobile machine. Therefore, the PUSC technique and the FUSC technique have a problem such that high-quality wireless communication cannot be expected even if the wireless propagation environment where the mobile machine is located is excellent.